There are a few IP addresses to consider.
In your home there is an IP address for each computer attached to your Belkin router, as well as one for the configuration page of the router itself.
Let's say your computer is 192.168.2.25, another computer at home is 192.168.2.26, and you get the configuration page by going to 192.168.2.1.
Your cable modem, or whatever you use to connect to the internet, has another IP address according to the outside world. Everything on your home network has this same IP as far as the outside world, and it is up to your router to sort or properly ROUTE all traffic from outside.
So, your computer thinks it's address is 192.168.2.25, the same address that you get in the System Prefs web sharing panel. However, http://www.whatismyip.com/
will report back to your real address according to the outside world. (Note, that this is not the same as the IP address for configuring your router)
For the sake of illustrating this, you can go to another computer on your home network and confirm that you get the same answer from Whatismyip.com.
So how does everything keep from getting confused? The router is essentially a small computer. When you type in an address, a request for data to be sent to your x.x.x.25 IP address is sent out. The router rewrites this to the real/outside IP address, then when the request is filled and the data begins to roll in it routes it to the same computer it knows asked for the data.
It is different when you are the website. Setting up the router to send all port:80 requests to the computer at x.x.x.25 makes all requests sent to your real/outside IP address will be forwarded to your computer.
I hope that makes sense, and that I didn't misunderstand your question.
If you'd rather not require people to memorize a changing IP address, you can also find free services that will forward a more conventional text-url to your computer.